Conventional solid polymer electrolysers operate using a proton exchange membrane (PEM). The membrane is in contact with a catalyst and electrode on each surface and the resulting membrane electrode assembly (MEA) is arranged as the separator between two chambers.
The chambers are normally filled with water and, when a voltage is applied to the electrodes, the water is dissociated to form a hydronium ion (H+) and a free oxygen ion (O−). Electrons entering the system from the external circuit facilitate the production of atomic oxygen (O) and thus molecular oxygen (O2), as bubbles in the water on the ‘oxygen’ side of the apparatus. The hydronium ion passes through the membrane to the ‘hydrogen’ chamber where it first forms, atomic hydrogen (H) and then molecular hydrogen (H2), which appears as bubbles in the water in the hydrogen chamber.
Water is necessary to ensure the continued operation of polymer membranes, as they lose ionic conductivity if they dry out. It is particularly necessary to have water present at the anode, as water is needed to form the oxygen ions, and thus oxygen gas.
An example of a conventional membrane is Nafion. Nafion has a relatively low water permeability, and therefore operates best when water is applied to both sides of the membrane.
The equipment needed to maintain the operation of an electrolyser is termed the “balance of plant” (BoP) and can represent a considerable fraction of the total cost of an electrolyser system. In a conventional system (FIG. 1), there are a number of necessary elements including:
(a) a water-gas separation tower connected to each electrode compartment;
(b) a heat exchanger connected to each electrode compartment;
(c) a water make-up system into the oxygen electrode compartment; and
(d) a water return system from the hydrogen electrode to the oxygen electrode, for water transferred by electro-osmotic drag.
All the above items must be pressure tested to the full operational pressure of the system and, in addition to the cost, the parasitic energy demand inherent in the pumping and circulation is high.
Some electrolysers may be inefficient, which can lead to the generation of heat. This can cause chemical and/or mechanical instability in the membrane. It is therefore necessary to ensure that there is an efficient cooling system in electrolyers.
WO03/023890 discloses hydrophilic polymer ion-exchange membranes. These membranes have increased water permeability and maintain hydration even in environments where little water is available.
WO2007/105004 discloses the use of composite hydrophilic polymer ion-exchange membranes in electrolysers. A method of electrolysis is disclosed, in which the hydrogen side of the membrane is predominantly free of water in liquid form. The membrane used in this method is a cationic exchange (CE) membrane.
WO2009/007691 discloses the electrolysis of seawater using hydrophilic membranes. The electrolysis of seawater using CE system with a dry cathode is disclosed.